Ball and socket joints are widely used in motor vehicles, particularly in automobile suspensions and steering linkages. Because of the relatively large number of joints that are used, considerable effort has been made to improve the effect of ball and socket joints on vehicle ride and handling characteristics.
Especially in a lightweight vehicle, ride and handling are improved if the vehicle's suspension or steering linkage offers a relatively small resistance to movements of the vehicle caused by small irregularities in a roadway. The bouncing or rebounding movements caused by large irregularities in a roadway, however, must be controlled by shock absorbers assisted by other components of the suspension and by the steering linkage. The joints, particularly ball and socket joints, incorporated in vehicle suspensions and steering linkages both permit and resist movements of the vehicle caused by roadway irregularities. Consequently, such joints should preferably offer both a relatively low resistance to small movements and a relatively high resistance to larger movements.
Joints previously proposed for vehicle suspensions or steering linkages have not provided dual levels of resistance to movements. For example, U.S. Pat. No. 3,667,789 describes and illustrates a ball and socket joint in which an annular frustospherical bearing insert is interposed between the outer frustospherical surface of a ball element and the adjacent inner frustospherical surface of a socket. The ball element is integral with a short stud that projects through the central opening in the bearing insert and through an aligned opening in the socket. The outer diameter of the stud is smaller than the diameter of the opening in the insert, which, in turn, is smaller than the diameter of the opening in the socket.
The ball and stud combination, or ball stud, of the '789 patent can rotate and tilt relative to both the bearing insert and the socket. In addition, the ball stud and insert can rotate and tilt together relative to the socket. Movement of the ball stud and insert together occurs when the stud strikes the edge of the insert during tilting or rotational movement. The insert may then be forced to move with the ball stud relative to the socket. Despite the possibility of relative motion between the ball stud and either the bearing insert and socket together or the socket alone, the resistance to movement remains constant regardless of the type of movement. This is because the socket, the bearing insert, and the ball element of the ball stud are all made of a ferrous metal. Consequently, the coefficients of friction and the resulting frictional forces or resistances to movement developed between abutting surfaces of the three components will all be substantially identical.
Another ball and socket joint that may be used in a vehicle steering or suspension system is described and illustrated in U.S. Pat. No. 3,238,602. A bearing liner or insert for the joint is laminated and includes a radially inner spherical layer of low friction material, a radially outer backing layer of relatively rigid material, and an intermediate layer of material for bonding the low friction material to the backing layer. The low friction layer of the bearing insert abuts the spherical outer surface of the ball of a ball stud. As a result, the frictional resistance to movement between the ball and the insert is relatively low. The backing layer of the insert, on the other hand, is made of metal and abuts a metal surface of the socket. Consequently, the frictional resistance to movement between the bearing insert and the socket is high.
Despite the inclusion of a laminated bearing insert with surfaces formed of different materials, the joint of the '602 patent will not provide two levels of resistance to movements. The patent intends that the bearing insert should not move relative to the socket and proposes welding as a method of positively securing the insert to the socket. Even if the insert is not welded to the socket, however, it is highly unlikely that movement will occur between the two components. At any point where the stud of the joint's ball stud contacts the edge of the bearing insert, the stud also contacts an adjacent edge of the socket. Tilting motion of the ball stud thus could not force movement of the insert relative to the socket. Without movement between the insert and the socket, there cannot be two different levels of resistance to movements permitted and experienced by the joint.
U.S. Pat. No. 703,899 describes and illustrates a ball and socket joint used to connect an arm or leg to the body of a doll. A spherically shaped bearing insert, which is made of an elastic material, is interposed between the ball and the socket of the joint. In one embodiment of the joint, the ball can tilt relative to the bearing insert and the socket together or the ball and insert can tilt together relative to the socket alone. Despite the two possible methods of accommodating tilting motion between the ball and the socket, there should be no difference between the resistances to movement. The coefficient of friction between the ball and the insert should be approximately the same as the coefficient of friction between the insert and the socket.